1. Field of the Invention
The present invention relates generally to a technique of measuring imaging characteristics of a projection optical system and, more particularly, to a measuring method suited to measure the imaging characteristics such as an asymmetric aberration like, e.g., a distortion or a comatic aberration of the projection optical system incorporated into a projection exposure apparatus employed in a lithography step for manufacturing a semiconductor element, a liquid crystal display element or a thin-film magnetic head, etc.
2. Related Background Art
In the lithography step for manufacturing a semiconductor element, a liquid crystal display element or a thin-film magnetic head, etc., there has been employed the projection exposure apparatus for imaging patterns on a photo mask or a reticle (hereinafter generally termed [reticle]) on a photosensitive substrate through the projection optical system. Generally, in the semiconductor element or the like, multi-layered circuit patterns are formed in superposition on a substrate. At the same time, for example, the projection exposure apparatus for exposing the circuit pattern of the first layer is different from a projection exposure apparatus for exposing the circuit pattern of the second layer. Accordingly, an overlay accuracy of the circuit patterns is kept high, and there is increased a yield of the semiconductor element or the like that is to be finally manufactured. For this purpose, it is required that a magnification error of the projection optical system and the imaging characteristics such as the distortion be maintained within predetermined standards. To fulfill this, the imaging characteristics of the projection optical system are at first required to be accurately measured. A known conventional measuring method is disclosed in, e.g., U.S. Pat. No. 4,629,313 and U.S. Pat. No. 4,908,656. Exemplifying the distortion as the imaging characteristic of the projection optical system, the measuring method thereof will hereinafter be explained.
A test reticle R1 as shown in FIG. 7 has hitherto been employed for measuring the distortion of the projection optical system. Referring to FIG. 7, a fiducial mark 2 is formed in the central portion of a test reticle R. The fiducial mark 2 consists of two-line-and-space patterns 1A, 1B arranged at a predetermined interval in an X-direction. Measuring marks 3A to 3D each consisting of a 3-line-and-space pattern are formed at four corners of the test reticle R1. An X-directional (measurement-directional) width of a dark or bright part of each of the fiducial mark 2 and the measuring marks 3A to 3D is on the order of 4 to 6 .mu.m. This width is set to a value optimized in accordance with a resolving power of a sensor for detecting images of these fiducial and measuring marks.
Then, when measuring the distortion, the measuring marks 3A to 3D on the test reticle R are illuminated with the light. In this state, the fiducial mark 2 is exposed (first exposure) on a wafer W through the projection optical system while sequentially moving the photosensitive substrate (e.g., the wafer coated with a resist) W. In consequence of this, as illustrated in FIG. 8A, latent images 2AP to 2DP of the fiducial mark 2 are formed on the wafer W. Forming positions of these latent images 2AP to 2DP coincide with projecting positions of the measuring marks 3A to 3D in FIG. 7 in terms of design on the wafer W. Next, the wafer W formed with the latent images 2AP to 2DP is positioned in a predetermined position. Thereafter, the images of the measuring marks 3A to 3D of the test reticle R1 are exposed (second exposure) in superposition on the water W through the projection optical system. As a result, the latent images 3AP to 3DP of the measuring marks are, as depicted in FIG. 8B, formed in between the latent images 2AP to 2DP respectively on the wafer W. Note that a dotted line in FIG. 8B indicates a projection region of the test reticle R1 when in the second exposure.
Next, the wafer W undergoing the first and second exposures is developed. The two latent images 2AP to 2DP of the fiducial mark 2 and the latent images 3AP to 3DP of the measuring marks are thereby formed in the shape of recessed or projected resist patterns on the wafer. These resist patterns can be viewed by use of, e.g., a microscope (or an SEM). FIG. 9A is an enlarged view showing the latent image 2DP of the fiducial mark and the latent image 3DP of the measuring mark in FIG. 8B. An X-directional difference .DELTA.x1 is measured between the center of the latent image 2DP and the center of the latent image 3DP. Similarly, there are measured differences between the centers of three sets of remaining latent images in FIG. 8B (e.g., an X-directional difference .DELTA.x4 between the center of the latent image 2AP and the center of the latent image 3AP). An X-directional distortion of the projection optical system can be thereby obtained. Note that the above difference is, as a matter of fact, obtained by detecting not the latent images but the resist patterns.
Next, one example of the measuring marks suitable for a conventional laser step alignment (LSA) method will be explained with reference to FIGS. 10A to 10C. The LSA method is disclosed in, e.g., U.S. Pat. No. 4,677,301, wherein when a diffraction grating mark is relatively scanned with an elongate band-like spot beam formed on the wafer, a beam of diffraction light generated from the mark is photoelectrically detected.
FIG. 10A illustrates a first measuring mark 5 formed on the reticle and corresponding to the fiducial mark 2 in FIG. 7. This first measuring mark 5 is formed such that square aperture patterns 5A, 5B, 5C, . . . each having one side dimensioned L are arranged at a pitch 2L in a direction (Y-direction) perpendicular to a measuring direction (X-direction) in a light shielding portion (chromium layer) 4. The width of each of the aperture patterns 5A, 5B, . . . is on the order of 4 to 6 .mu.m. FIG. 10B also illustrates a second measuring mark 7 corresponding to the measuring marks 3A to 3D in FIG. 7. This second measuring mark 7 is also formed such that square aperture patterns 7A, 7B, 7C, . . . each having the width L are arranged at the pitch 2L in the Y-direction in a light shielding portion.
Then, the first and second measuring marks 5, 7 are sequentially exposed on the wafer, and developing thereof is effected essentially by the same actions as the above-mentioned. As depicted in FIG. 10C, first and second recessed measuring mark images 5P, 7P are formed in a positive type resist 8. If the resist is of a negative type, however, the first and second measuring mark images 5P, 7P are formed as projected portions. Next, as illustrated in FIG. 10C, a spot beam 9 extending in the Y-direction falls on the wafer after being developed. In this state, the wafer is scanned in the X-direction. Note that an X-directional width of the spot beam 9 is approximately equal to the X-directional width of the measuring mark image 5P or 7P. When the measuring mark image 5P or 7P coincides with the spot beam 9 in the middle of scanning, the intensive diffraction light is generated in a predetermined direction. It is therefore possible to detect an X-directional position in which the measuring mark image 5P or 7P coincides with the spot beam 9. With this detection, there is obtained an X-directional interval .DELTA.x from the first measuring mark image 5P existing in an ideal position in terms of design to the second measuring mark image 7P. This interval .DELTA.x corresponds to a distortion of the projection optical system.
Next, one example of a measuring mark by the conventional Moire method will be explained with reference to FIGS. 11A to 11C. Turning to FIG. 11A, light transmitting (or shielding) patterns 10A, 10B, 10C, . . . each assuming a triangular waveform are arranged at a pitch Q1 in the Y-direction on the reticle. These patterns form the first measuring mark corresponding to the fiducial mark 2 in FIG. 7. Further, for light transmitting (or shielding) patterns 11A, 11B, 11C, . . . each assuming the triangular waveform are arranged at the pitch Q1 in the Y-direction. These patterns form the second measuring mark corresponding to the measuring marks 3A to 3D in FIG. 7. The pitch Q1 is substantially equal to the pitch of the measuring mark 3A in FIG. 7. The patterns 11A, 11B, 11C . . . are shown in superposition on the first measuring mark in order to facilitate understanding of FIGS. 11A and 11B. As a matter of fact, the first and second measuring marks are formed in portions different from each other on the reticle.
The first and second measuring marks are sequentially exposed on the wafer, thus performing the development. Then, as illustrated in FIG. 11B, the overlaid portions of the first and second recessed or projected measuring marks are left as overlaid images 12P consisting of rhombic patterns 12AP to 12EP in the resist film. Let Lx be an X-directional length of each of the rhombic patterns 12AP to 12EP. This length Lx is obtained by enlarging a Y-directional crosswise deviation quantity between the first and second measuring mark images in FIG. 11A in accordance with the principle of Moire fringes. It is therefore possible to highly accurately measure the Y-directional crosswise deviation quantity between the first and second measuring mark images, i.e., a Y-directional distortion by measuring differences Lx1, Lx2 of the positions thereof from a detection signal S shown in FIG. 11C.
Referring further to FIG. 11B, overlaid images 13P, 14P consisting of rhombic patterns are similarly formed in the X-direction with respect to the overlaid images 12P. Then, for measuring the X-directional length Lx of each rhombus, the wafer is scanned in the X-direction in a state where the spot beam 9 extending in the Y-direction falls on the wafer after being developed. If the overlaid images 12P to 14P are overlapped with the spot beam 9 in the middle of scanning, the diffraction light is generated in the predetermined direction. For this reason, it is feasible to detect the X-directional positional differences Lx1, Lx2 with which the respective overlaid images 12P to 14P are overlapped with the spot beam 9. A distortion quantity .DELTA.Y=(Lx2-Lx1)/k can be measured at a high accuracy by using values of Lx1, Lx2 and an enlargement magnification k of the Moire mark. The measuring principle by the Moire method is disclosed in U.S. patent application Ser. No. 984,558 (filed on Dec. 2, 1992).
Next, there will be explained one example of the measuring mark by a conventional box-in-box method with reference to FIGS. 12A to 12C. The box-in-box method is a method capable of simultaneously measuring X- and Y-directional positional deviation quantities through a pair of marks. FIG. 12A illustrates a first measuring mark 15 corresponding to the fiducial mark 2 of FIG. 7 on the reticle. This first measuring mark 15 is a square pattern consisting of a light shielding portion (chromium layer). FIG. 12B also illustrates a second measuring mark 16 corresponding to the measuring marks 3A to 3D in FIG. 7. This second measuring mark 16 includes a square aperture (light transmitting) pattern 17 formed at the center of the square light shielding pattern. Widths of the first measuring mark 15 and of the aperture pattern 17 at the center of the second measuring mark 16 take values each larger than 4 to 6 .mu.m.
The first and second measuring marks 15, 16 are sequentially exposed on the wafer, thus effecting the development thereof. Then, as shown in FIG. 12C, edges of the resist film 18 are formed with an edge portion 15P of the first measuring mark image and an edge portion 17P of the image of the aperture pattern 17 at the center of the second measuring mark. Subsequently, these measuring mark images are scanned with the spot beam in the X- and Y-directions. It is thereby possible to obtain a positional deviation quantity .DELTA.x between the X-directional center of the edge portion 15P and the X-directional center of the edge portion 17P. Obtained also is a positional deviation quantity .DELTA.y between the Y-directional center of the edge portion 15P and the Y-directional center of the edge portion 17P. The X- and Y-directional distortions of the projection optical system can be obtained respectively from these positional deviation quantities .DELTA.x, .DELTA.y.
As discussed above, in the measuring marks or the fiducial mark for the distortion of the conventional projection optical system, the width of the light shielding or transmitting portion is set to 4 to 6 .mu.m in accordance with a resolving power, etc. of a sensor for detecting the image projected on the photosensitive substrate. In contrast with this, the width of the light shielding or transmitting portion of the pattern is going to be smaller than 0.5 .mu.m, the pattern being such as a reticle circuit pattern or the like (hereinafter called [actual element pattern]) which is conceived as an actually transferred object. For this reason, a characteristic measured with the mark for measuring the distortion differs from the distortion characteristic with respect to the actual element pattern. This leads to such a drawback that the distortion with respect to the actual element pattern cannot be accurately measured.
It can be also considered that the pitches of, e.g., the measuring mark images 2DP, 3DP in FIG. 9A are simply reduced for measuring the characteristic corresponding to the actual element pattern, and, as illustrated in FIG. 9B, a fiducial mark image 20P and a measuring mark image 21P are formed. It is, however, difficult for the conventional sensor to measure the whole positional deviation quantities by measuring positions of the rectilinear patterns as done for such hyperfine patterns.
Given next is an explanation of a difference between the imaging characteristics when a line width of the fiducial or measuring mark is different from a line width of the actual element pattern.
FIG. 13A shows a test reticle R1 formed with a measuring mark 3A. FIG. 13B illustrates a reticle R2 formed with an actual element pattern 19. In this case, it is assumed that the measuring mark 3A is a line-and-space pattern in which a width of the light shielding or transmitting portion is 4 to 6 .mu.m. The actual element pattern 19 is, it is also assumed, a line-and-space pattern in which a width of the light shielding or transmitting portion is 0.5 .mu.m. Further, a diffraction angle .theta. of nth-order diffraction light coming from the pattern satisfies the following equation: EQU p.multidot.sin .theta.=n.multidot..lambda.
where .lambda. is the wavelength of the exposure light IL with which the reticle is illuminated, and p is the pitch of the illuminated pattern.
Namely, the diffraction angle .theta. of the diffraction light of the same order increases with a more infinitesimal pattern pitch. The diffraction light travels through a position far from the optical axis on the pupil plane of the projection optical system. Accordingly, the diffraction light from the measuring mark 13A in FIG. 3A passes through a position close to the optical axis on the pupil plane of the projection optical system. The diffraction light from the actual element pattern 19 in FIG. 13B passes through a position apart from the optical axis on the pupil plane of the projection optical system. If surface configurations and refractive indexes of optical members constituting the projection optical system deviate from ideal values, a variety of aberrations are caused due to a difference between the positions through which the light passes on the pupil plane. Especially when a comatic aberration remains as it is, an imaging position, as shown in FIG. 13C, differs corresponding to the passage positions on the pupil plane of the projection optical system PL. As a result, the distortion differs. For this reason, it follows that the characteristic measured with the measuring mark 3A is different from the characteristic with respect to the actual element pattern 19.
In the case of using a normal illumination method in which a conventional coherence factor (.sigma.-value) is on the order of 0.5, as illustrated in FIGS. 14A and 14B, both of diffraction light DL+ and diffraction light DL- from the patterns on the reticles R1, R2 contain the beams of light passing in the vicinity of the optical axis of the projection optical system PL. Hence, the diffraction light from the measuring mark having a larger pattern pitch as shown in FIG. 14A and also the diffraction light from the actual element pattern having a smaller pattern pitch as shown in FIG. 14B respectively travel through an extremely wide range on the pupil plane of projection optical system PL. Therefore, a difference between the imaging characteristics is quite small.
Contrastingly, in the case of using a phase-shift method with a small coherence factor of the illumination or a transform illuminant method (oblique illumination method) of restricting the illumination light passing through the pupil plane of the illumination optical system in at least one segmented area eccentric from the optical axis, the diffraction light from the reticle pattern contains almost no component traveling in the vicinity of the optical axis of the projection optical system. Consequently, there is produced a large difference between the imaging characteristics of the projection optical system depending on the pattern pitches. FIG. 15A illustrates how a phase-shift reticle R3 formed with a large-pitch pattern is illuminated with a beam of light. FIG. 15B shows how a phase-shift reticle R4 formed with a fine-pitch pattern is illuminated with the beam. In this case, the passage positions of the diffraction light DL+ and the diffraction light DL- on the pupil plane of the projection optical system PL, differ depending on the pitches. That is, a state of aberration differs. A difference between these pattern imaging positions is also caused. This is substantially the same as employing the transform illuminant method. Note that the transform illuminant method is disclosed in, e.g., U.S. patent application Ser. No. 791,138 (filed on Nov. 13, 1991) and U.S. patent application Ser. No. 847,030 (filed on Apr. 15, 1992).
To summarize, when using the projection optical system PL in which, for instance, the asymmetric aberration such as a comatic aberration exists, it is impossible to accurately obtain the distortion with respect to the actual element pattern having a small pitch by measuring the distortion through the conventional large-pitch measuring pattern. Consequently, there arises a drawback of inducing a decline of the overlay accuracy when forming the multi-layered circuit patterns on the photosensitive substrate.
Further, when checking the asymmetric aberration such as the comatic aberration, etc., among the imaging characteristics of the projection optical system, the check pattern such as a line-and-space pattern has hitherto been printed on the wafer. Then, an asymmetric quantity of a resist image of the check pattern formed on the wafer by the developing process is measured by use of a scan type electronic microscope (SEM) or the like. A comatic aberration quantity of the projection optical system is calculated from this measured result. However, measuring the resist pattern by a high-accuracy measuring apparatus separate from the projection exposure apparatus presents such drawbacks that the whole measuring system is complicated and expensive, and the measurement of the aberration is very time-consuming. In addition, for measuring the asymmetric aberration of the projection optical system, it is desirable that the number of measuring points in an exposure field of the projection optical system be increased as much as possible. It is, however, difficult to increase the number of measuring points, if the measurement takes much time as in the prior art.